Formation of thin gate insulators with high electrical characteristics is a key technology to achieve high performance of semiconductor devices in the next generation. As for thin film transistors (TFTs) used in liquid crystal displays (LCDs), a new low temperature gate oxide formation technique is required to provide homogeneous gate insulators on rough poly-Si surfaces with substrate glass. We have developed two different methods for nitric acid oxidation of Si (NAOS) at 120^oC: i) one-step NAOS using azeotropic nitric acid solutions (68wt%) to form ultrathin (i.e., ∼1 nm) SiO₂ layers with an extremely low leakage current density, and ii) two-step NAOS using ∼40 and 68wt% nitric acid solution to form thick (i.e., ≥ 10 nm) SiO₂ layers. A progress in the NAOS techniques is outlined in this report.